As information networks evolve rapidly, fiber optic networks become more and more important. The fiber optic network provides not only low transmission loss but also high transmission capacity. In a DWDM communication system, 16, 32, and even more channel wavelengths can be transmitted in a single optical fiber. In order to guarantee the quality of transmission signals in a fiber optic network system, it is necessary to effectively monitor optical power, optical wavelength, and OSNR. Wherein, the OSNR is the most important parameter to be monitored. The importance of the OSNR stems from the fact that it represents the degree of signal impairment after an optical signal is transmitted through an optical amplifier of an optical communication system. The attenuation and dispersion of an optical signal will affect the detectability of the optical signal. An amplified spontaneous emission (ASE) noise, which is first produced by an optical amplifier and then received by a receiver, may result in a significant reduction of the transmission efficiency of an optical communication system. In practical applications, the use of optical amplifiers may improve communication quality due to an enlargement of signal amplitude. However, the noise accompanied with the optical signal is amplified as well. The end result is a deterioration of the OSNR.
In order to accurately measure OSNR, it is required to carefully design a detector module. For example, conventional approaches using polarization rotators and linear polarizers or adopting various combinations of different optical filters to improve the accuracy of noise measurements have been proposed. However, the schemes that use polarization-selective components can be easily influenced by polarization fluctuation during signal transmission. For most optical filtering approaches, it is generally difficult to use a single tunable optical filter (TOF) for both optical signal power and noise power measurements.
In 2000, Chappel et al. (“Optical signal-to-noise ratio characterization demands precision and flexibility,” WDM Solutions, vol. 2, no. 6, pp. 55-60, November 2000) proposed a method for accurately measuring the optical signal power and the noise power. The method requires an optical spectrum analyzer (OSA) with a wide enough resolution bandwidth (RBW) to accommodate an optical signal modulation sideband. Furthermore, the dynamic range of the OSA must be large enough to reject adjacent optical channels. As for the measurement of noise power, it can be performed by measuring two noise components at both sides of the optical channel in the optical spectrum. Then, the noise component mixed in the optical channel is obtained by an interpolation between the two measured noise components. Alternately, a dual sweep technique can be used for the OSNR measurement. This technique adopts an optical filter with a wide RBW to measure the optical signal power, and another optical filter with a narrow RBW to measure the noise power. Its drawback is an increase of the complexity due to the use of a dual sweep and the requirements of a wide RBW optical filter and a narrow RBW optical filter.
In 1998, Hentschel et al. (“Fiber Optic Test and Measurement”, Prentice-Hall, 1998, pp. 101-115) applied a double-pass filtering technique to OSAs. The double-pass filtering technique was achieved by accurate tuning control of high-precision optical elements. Incoming and outgoing lights are directed to different trajectories for separate coupling. Though this technique is also adopted to increase the dynamic range of an OSA, it usually requires a sophisticated tuning control and optics. Besides, it occupies more space than that required for other techniques.
In business applications, Fabry-Pert etalon has been widely used in single wavelength distributed feedback lasers to control the wavelength. Therefore, its requirements of wavelength monitoring are not very strict. The location of each optical channel can be clearly identified by scanning and filtering the optical spectrum with a TOF. In DWDM networks, it is very important to maintain the quality of optical channel signals. And, effectively analyzing the quality of optical channel signals is a foundation for the maintenance of an optical communication system to guarantee the quality of the system.
The conventional techniques mentioned above for monitoring OSNR, a dual sweep technique and a double-pass filtering technique applied to OSAs, will result in an increase of both complexity and cost. The present invention provides an apparatus and a method for monitoring OSNR to not only lower the complexity and cost of the monitoring system but also increase the sensitivity of the monitoring system.